Described below is a method of reducing the transmission time interval (TTI) in enhanced general packet radio service (GPRS) EGPRS networks.
Currently, radio blocks are divided into four bursts; up to and including in 3GPP GERAN Release 6 the bursts are transmitted on a specific timeslot in four time division multiple access (TDMA) frames. Each TDMA frame is approximately 5 ms in duration, making the transmission time interval approximately 20 ms.
According to the reduced transmission time interval (TTI) RTTI scheme introduced in Release 7 of the 3rd Generation Partnership Project (3GPP) General Packet Radio Service (GPRS)/Enhanced Data for global system for mobile communication (GSM) Evolution (EDGE) Radio Access Network (GERAN) 3GPP GERAN standards, timeslots (of which there are eight, numbered 0 through 7, per TDMA frame) can be combined into pairs; then four bursts are transmitted using two (paired) timeslots in each of two TDMA frames, reducing the TTI to approximately 10 ms. The configuration of a pair of timeslots (which may or may not be on the same carrier) is referred to as a packet data channel (PDCH)-pair.
In the case of downlink dual carrier assignments a mobile can receive on two different carriers simultaneously. This addresses both the case where PDCH-pairs must use timeslots on the same carrier and the case where no such restriction exists (where so-called ‘split PDCH-pairs’ are possible). In the latter case, this concerns only the case where, if PDCH-pairs are on different carriers, the timeslot numbers are the same.
In general, this considers that PDCH pairs cannot ‘split’ another pair e.g. a pair on 1,3 cannot co-exist with a pair on 2,4.
It is assumed that in any RTTI configuration, there must be at least one uplink (UL) PDCH pair and one downlink (DL) PDCH pair (even though it is not essential that the mobile has resources assigned on both, they are needed for packet associated control channel (PACCH)).
Due to the medium access control (MAC) protocols used in GERAN, it is not required that every UL PDCH pair must have a corresponding DL PDCH pair (in such a case, either extended dynamic allocation (EDA) can be used, or a modified shifted uplink status flag (USF) approach can be used see 3GPP TS 44.060 v.7.0.0).
Similarly, it is not required that every DL PDCH pair must have a corresponding UL PDCH pair: the network is forbidden from polling a mobile on a DL PDCH pair for which no corresponding UL PDCH pair exists.
The problem here is to communicate efficiently to the mobile which timeslots are configured as PDCH-pairs and on which of these PDCH-pairs the mobile is assigned resources.
A subset of this problem is to efficiently encode the description of how PDCH-pairs are assigned to different timeslots and, in the case of Downlink Dual Carrier, different carriers.
It is further necessary to specify how a mobile determines, based on the configuration and/or its assignment, which uplink (UL) and downlink (DL) PDCH-pairs correspond to each other. These so-called ‘corresponding pairs’ govern, for example, on which DL PDCH-pair the mobile should expect to receive an uplink state flag (USF) indicating that it may transmit on a given UL PDCH-pair; also, on which UL PDCH-pair should a mobile respond to a poll sent on a given downlink PDCH-pair.
Description of PDCH-pairs is separated into a “configuration” and an “assignment”. A mobile may receive a message which describes a change in configuration or assignment or both.
Broadly speaking, the configuration describes all RTTI PDCH-pairs currently in use on a given carrier (or pair of carriers), and is distinct from timeslots which are used for non-RTTI packet transfer, or for circuit-switched voice or data transfer. The assignment describes the subset of PDCH-pairs on which a given mobile can expect to transmit or receive data, and also specifies various RLC and MAC parameters (uplink state flag, etc.).
Two methods of encoding the configuration are specified using a bitmap, either fixed or variable length, depending on whether or not ‘split PDCH-pairs’ are permitted.
Rules for determining which PDCH-pairs are ‘corresponding PDCH-pairs’ are specified, based on either the configuration or the assignment.
Rules for informing the mobile of a change in assignment or a change in configuration, or both are specified, including the definition of a new message to inform mobiles (possibly a broadcast message) that a configuration has changed, but the assignment remains (broadly) as before, based on the new configuration.
A mobile may be sent a message describing a modification to an existing assignment, without a corresponding change in configuration, e.g. a downlink assignment message may indicate that the mobile now is assigned resources on downlink PDCH-pairs 1 & 2, instead of on 2 & 3.
A mobile may be sent a message describing a modification to an existing configuration, without a corresponding change in assignment, e.g., downlink PDCH-pair 3 now uses timeslots 5 & 6, rather than on 5 & 7. Any resources assigned on DL PDCH-pair 3 remain unchanged (but using the new timeslots).
Since the configuration affects all mobiles using resources on a specific carrier, a message indicating a change of configuration is defined which is a broadcast message. That message may be sent repeatedly to ensure reception and may include a ‘start time’ at which the new configuration will apply.
The mobile should be informed of the configuration currently being used. The configuration need not be described in subsequent assignment messages if it has not changed.
PDCH-pairs within a configuration are identified by a number, and the identifying number is increased in order of the timeslot numbers used in the configuration. E.g., PDCH-pair using timeslots 1 & 2 is PDCH-pair number 1, PDCH-pair on timeslots 3 & 5 is PDCH-pair number 2. In order to re-use existing message structures, these identifying numbers are considered equivalent to timeslot numbers in legacy configurations—legacy messages/structures etc. referring to timeslot numbers can be used without change, to refer to PDCH-pairs.
A default configuration is specified, having 4 PDCH-pairs per carrier, with PDCH pair i (0≦i≦3) using timeslots 2i and 2i+1, both in the UL and DL.
The described PDCH-pair configuration need not match the actual PDCH-pair configuration, provided that assignments refer only to PDCH-pairs which actually exist. This means that, for example, the default configuration may be indicated even if only a subset of PDCH-pairs in the described configuration exist. This has the benefit of reducing the amount of signaling information both for the default configuration and where the actual configuration changes but, thanks to this rule, no actual notification is required to mobiles.